1. Field of the Invention
The present invention relates to processing substrates in processing chambers, such as a semiconductor processing reactor, and particularly to a method of increasing throughput for process steps at controlled temperatures.
2. Description of the Related Art
High-temperature ovens, called reactors, are used to create fine dimension structures, such as integrated circuits, on semiconductor substrates. Several substrates, such as silicon wafers, are placed on a wafer support inside the reactor. Both wafer and support are heated to a desired temperature. In a typical wafer treatment step, reactant gases are passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of the reactant material on the wafer. A series of such treatment steps on a single wafer is called a recipe. If the deposited layer has the same crystallographic structure as the underlying silicon wafer, it is called an epitaxial layer. This is also sometimes called a monocrystalline layer because it has only one crystal structure. Through subsequent deposition, doping, lithography, etch and other processes, these layers are made into integrated circuits, producing from tens to thousands or even millions of integrated devices, depending on the wafer size and the circuits' complexity.
Various process parameters are carefully controlled to ensure the high quality of the resulting layers. One such critical parameter is the wafer temperature during each recipe step. During CVD, for example, the deposition gases react within particular temperature windows and deposit on the wafer. Different temperatures also result in different deposition rates. Accordingly, it is important to accurately control the wafer temperature to bring the wafer to the desired temperature before the treatment begins.
One factor which critically affects the throughput of a CVD processing reactor is the wafer temperature ramp rate. Such temperature ramping can be required at several points during a given recipe. For example, a cold wafer must be heated to the appropriate treatment temperature. Also, the recipe may require different temperatures for different treatment steps. At the recipe's end, the wafer ordinarily is cooled to a level that the wafer handling device can tolerate. The heating and cooling steps can represent a significant percentage of the processing time and can limit the reactor's throughput. The time between the steady state temperatures is essentially time which should be minimized so as to increase the reactor's throughput.
The rate at which the wafer temperature can change from one steady state to another depends on the reactor's ramp rate. The reactor's ramp rate depends on the temperature controller type, temperature sensor, energy source, and other process considerations. A thermocouple is a device for measuring temperature in which a pair of wires of dissimilar metals (e.g., copper and iron) is joined and the wire's free ends are connected to an instrument (e.g., a voltmeter) that measures the difference in potential that is created at the junction of the two metals. When thermocouples are used to measure the wafer temperature, the thermocouple's thermal mass limits the response time to temperature changes. Thus, during a ramp, the thermocouple measurement significantly lags the wafer temperature. Reactors employing thermocouples are typically operated at ramp rates slower than the heating mechanisms can handle to limit the temperature difference between the wafer and thermocouple. If the ramp rate is too high, such that by the time the thermocouple temperature catches up to the wafer temperature the wafer has been at a significantly higher temperature, the temperature controller reacts after the wafer temperature overshoots the target temperatures.
An optical pyrometer determines the wafer temperature by measuring the amount of light emitted by the wafer. The pyrometer reacts to temperature changes faster than the thermocouple and therefore does not significantly lag the wafer temperature. Therefore, a pyrometer can be used as a temperature controller sensor element to rapidly respond to changes in wafer temperature. Rapid response time allows for higher ramp rates without overshooting the target temperature, which can minimize the overhead time between steady state temperatures. The increase in ramp rate can significantly increase the reactor's throughput.
While optical pyrometers demonstrate response time advantages over thermocouples, pyrometers are less accurate in calibration readings. Among other problems, wafer emissivity changes at different temperatures, and with different structures at the surface, such that accurate calibration over a temperature range, and at different stages of integrated circuit processing, is challenging and is difficult to achieve with accuracy. Moreover, pyrometer instrumentation itself tends to alter readings over time, all other things being equal.
Both emissivity differences and drift call for initial and periodic calibration of the pyrometer with more accurate temperature measuring devices such as thermocouples. Typically, an instrumented or sacrificial sample wafer is utilized for this purpose. Not only is the instrumented wafer itself expensive, but down time required for calibration cycles translates to reactor down time and lower wafer throughput, leading to higher production cost.
U.S. Pat. No. 4,854,727 (the '727 patent) discloses an exemplary method for calibrating the emissivity characteristics of a semiconductor wafer temperature measurement element. The '727 patent discloses comparing the temperature measured within a susceptor in close proximity to the center of the wafer with the temperature measured by a radiation pyrometer by using a sample wafer, prior to the processing of a batch of similar wafers. The temperature measurements for the wafers in the batch are corrected with reference to the measurements taken by using the sample wafer.
Accordingly, a need exists for an apparatus and method for controlling wafer temperatures that avoids the slow response time of thermocouples and the inaccuracy and calibration down time associated with optical pyrometers.